Related Issues:

• [pigeon] Support @Sendable in Swift code #140439
• [pigeon] Support Swift Concurrency style api #147283

Use case

Sendable was introduced in Swift 5.5 to ensure thread-safe interactions in concurrent code. With the Swift 6 language mode in Swift 6.0 (Xcode 16), stricter conditions for Sendable conformance are enforced. Although newer Swift compiler can compile in Swift 5 mode, many developers prefer the stricter safety guarantees of Swift 6.

Pigeon, a code generator for Dart and native platform communication, enables seamless cross-platform development. However, the PigeonError class currently does not conform to Sendable, leading to compilation errors in Swift 6 mode and blocking developers from adopting the latest Swift concurrency features.

Example error:

Stored property 'details' of 'Sendable'-conforming class 'PigeonError' has non-sendable type 'Any?'

Proposal

The current PigeonError class is a simple implementation of the Error protocol:

/// Error class for passing custom error details to the Dart side.
final class PigeonError: Error {
  let code: String
  let message: String?
  let details: Any?

  init(code: String, message: String?, details: Any?) {
    self.code = code
    self.message = message
    self.details = details
  }

  var localizedDescription: String {
    return "PigeonError(code: \(code), message: \(message ?? "<nil>"), details: \(details ?? "<nil>"))"
  }
}

To make the class compatible with Swift 6.0 mode, I propose changing the details property type from Any? to Sendable?. This ensures the class conforms to Sendable since:

• The Error protocol already conforms to Sendable.
• All other properties (String and Optional) are Sendable.

This change introduces backward incompatibility because of the details type change. However, most existing use cases of PigeonError involve small, well-defined payloads, so migration should be straightforward. Developers can update their code by ensuring objects passed to details conform to Sendable.